1. Field of the Invention
The invention relates to a negative electrode active material for a lithium secondary battery that contains tin and carbon, a lithium secondary battery using the negative electrode active material, and a method of manufacturing the negative electrode active material.
2. Description of the Related Art
Carbon material, such as graphite, has been widely used as a negative electrode active material used in lithium secondary batteries. However, in general, the amount of Li that can be adsorbed by carbon material is small and therefore, focus is now put on Sn that adsorbs a larger amount of Li as compared to carbon material. Sn has the theoretical capacity of 994 mAh/g. Thus, Sn is relatively inexpensive material because, as compared to the practically used graphite that has the theoretical capacity of 372 mAh/g, Sn has relatively high theoretical capacity, that is, the theoretical capacity per unit weight of Sn is 2.7 times as large as that of graphite, and the theoretical capacity per unit mass of Sn is 8.9 times as large as that of graphite.
However, the volume expansion and contraction of Sn during adsorption and desorption of lithium are large. Thus, when charge and discharge, are performed in a lithium secondary battery in which a negative electrode active material containing Sn is used, Sn repeats expansion and contraction during adsorption and desorption of Li and as a result, the negative electrode active material cracks. When change and discharge are further repeated in this condition, the negative electrode active material cannot withstand drastic expansion and contraction of Sn and therefore falls off the current collector. As a result, electrical conductivity becomes insufficient and charge and discharge become impossible, so that the negative electrode loses its functionality and the cycle characteristics are deteriorated.
In order to solve the above problem, in Japanese Patent Application Publication No. 2003-151543 (JP-A-2003-151543), a negative electrode active material is proposed that contains a negative electrode active material substance containing Sn and carbonaceous matter and in which the carbonaceous matter is covered with the negative electrode active material substance. In the negative electrode active material, carbonaceous matter that exhibits relatively small expansion and contraction caused by charge and discharge is used for cores, so that the stress that occurs because of expansion and contraction is distributed, the negative electrode active material is prevented from being pulverized, and therefore, the cycle characteristics are improved. JP-A-2003-151543 also proposes a negative electrode active material in which the carbonaceous matter is formed into cores and the cores are covered with the negative electrode active material substance and on top of that covered with an electrically conductive material. In this case, the electron conductivity of the negative electrode active material is improved and the increase in contact resistance between the negative electrode active material particles due to expansion and contraction caused by charge and discharge is suppressed, so that the cycle characteristics are improved. However, the negative electrode active material substance nevertheless cracks and falls off the current collector due to expansion and contraction caused by charge and discharge and therefore, the cycle characteristics are not sufficient.
Japanese Patent Application Publication No. 2004-200003 (JP-A-2004-200003) proposed the idea of covering the surface of the negative electrode active material layer containing Li and Sn with carbon material. In this way, the surface of the negative electrode active material layer is electrically uniformized and it is possible to improve the cycle characteristics. However, it cannot be achieved by this method to prevent the negative electrode active material from cracking or falling off the current collector due to expansion and contraction caused by charge and discharge.
Japanese Patent Application Publication No. 2005-347076 (JP-A-2005-347076) proposes a negative electrode active material in which a low melting point alloy and carbon material adhere to the surface of Sn alloy particles that are made amorphous. With this negative electrode active material, it is possible to prevent the negative electrode active material from being pulverized and secure electrically conductive paths. In Japanese Patent Application Publication No. 2004-178922 (JP-A-2004-178922), a negative electrode material is proposed that contains a mixture of particles containing Sn and vapor-grown carbon fibers. JP-A-2004-178922 also describes a technology for covering the surface of particles containing Sn with carbonaceous matter containing a polymer, such as phenol resin. With this negative electrode material, it is possible to improve the electrical conductivity and the cycle characteristics. However, even with these technologies, sufficient improvement in the cycle characteristics is not achieved.